The present invention relates generally to suspension systems for isolating and reducing the transmission of vibratory motion between an object or payload and a base and, more particularly, to a compact horizontal-motion vibration isolator and system which can be made lower in vertical height while at the same time making it less sensitive to changes in weight to effectively reduce the transmission of horizontal vibrations between the object and the base. An isolator made in accordance with the present invention provides low frequency isolation and provides high levels of vibration isolation performance while offering a physical form factor that is easy to integrate into instrumentation setups.
The problems caused by unwanted vibration on equipment, devices and processes that are extremely motion sensitive have been widely researched and numerous solutions to prevent or reduce the transmission of vibratory motion have been proposed and developed. Many of the devices designed to reduce the transmission of unwanted vibration between an object and its surroundings, commonly called vibration isolators or suspension devices, have utilized various combinations of elements such as resilient pads made from a variety of materials, various types of mechanical springs, and pneumatic devices. There are, however, shortcomings and disadvantages associated with these particular prior art isolation systems which prevent them from obtaining low system natural frequencies and from limiting internal structural resonant responses to low values while providing high isolation performance at the higher frequencies.
Current methods for horizontal-motion isolation include:
1) Pendulums. These devices support the payload by hanging it from a set of rods or cables. The pendulums, and hence, the mechanical path, must be sufficiently long to achieve a low frequency and tend be fairly complicated.
2) Inverted pendulums, or columns. These can be made short, but exhibit strong sensitivity to payload changes as the critical buckling load is approached. When the columns/inverted pendulums are made shorter in length, the more sensitive they become.
3) Springs. Self-standing and stable springs can be used to support payloads vertically while giving vertical and horizontal isolation. However, it is often difficult to get a stable spring that also has low lateral stiffness.
4) Ball bearing between shallow concave disks. These mechanisms can be made very short, but offer limited performance at low amplitudes as they can be locked by frictional forces if there is not sufficient amplitude to overcome static friction acting on the mechanism.
5) Rubber bellows supporting gimble pistons. These components are found on many air tables used for vibration isolation. However, it is often difficult to obtain low frequencies utilizing these devices since they usually rely on a rubber bellow membrane made stiff by air pressure operating in a shear and rolling manner.
6) Elastomeric pads. These operate much like self-stable, standing springs. However, they cannot achieve low resonant frequencies in a horizontal direction very easily.
These components/methods for effecting horizontal-motion isolation have limitations which the current invention addresses.
Novel vibration isolation systems devices which utilize negative stiffness elements to reduce the stiffness of supporting columns and a support spring are described in U.S. Pat. No. 5,530,157, entitled “Vibration Isolation System” issued May 10, 1994, U.S. Pat. No. 5,370,352, entitled “Damped Vibration System” issued Dec. 6, 1994, U.S. Pat. No. 5,178,357, entitled “Vibration Isolation System” issued Jan. 12, 1993, U.S. Pat. No. 5,549,270, entitled “Vibration Isolation System” issued Aug. 27, 1996, U.S. Pat. No. 5,669,594, entitled “Vibration Isolation System” issued Sep. 23, 1997, U.S. Pat. No. 5,833,204, entitled “Radial Flexures, Beam-Columns and Tilt Isolation for a Vibration Isolation System issued Nov. 10, 1998, and U.S. Pat. No. 9,261,155, entitled improved Vibration Isolation Systems, Serial Number issued Feb. 16, 2016, which are all hereby incorporated by reference in this present application. These vibration isolators exhibit low stiffness, high damping to limit resonant responses of the composite system, effective isolation at the higher frequencies, and can provide high isolator internal structural resonant frequencies.
The particular vibration isolation systems described in these patents provide versatile vibration isolation by exhibiting low stiffness in an axial direction (generally the direction of the payload weight) and any direction substantially transverse to the axial direction (generally a horizontal direction), and may provide tilt or rotation about three mutually perpendicular axes. The present invention, however, is directed only to isolators used to isolate vibratory motion in the horizontal direction. It should be appreciated, however, that the present invention could be connected in series with a vertical-motion isolator and/or a tilt-motion isolator to provide bi-directional or omni-directional isolation as well. In subsequent discussions, an isolator which isolates vibrations in any direction substantially transverse to the direction of the payload will be referred to as a horizontal-motion isolator, and a system using multiple horizontal-motion isolators will be referred to as the horizontal-motion isolation system.
In the embodiments described in the above-noted patents, the isolators rely on a particular principle of loading a particular elastic structure which forms the isolator or a portion of it (the loading being applied by either the supported weight or by an external loading mechanism) to approach the elastic structure's point of elastic instability. This loading to approach the point of elastic instability, also called the “critical buckling load” of the structure, causes a substantial reduction of either the vertical or the horizontal stiffness of the isolator to create an isolation system that has low stiffness in the vertical and in any horizontal direction, and increases the damping inherent in the structure. While stiffness is reduced, these isolators still retain the ability to support the payload weight.
In the event that the load on the elastic structure is greater than the critical buckling load, the excessive load will tend to propel the structure into its buckled shape, creating a “negative-stiffness” or “negative-spring-rate” mechanism. By combining a negative-stiffness mechanism with a spring, adjusted so that the negative stiffness cancels or nearly cancels the positive stiffness of the spring, one obtains a device that can be placed at or near its point of elastic instability. The magnitude of the load causing the negative stiffness can be adjusted, creating an isolator that can be “fine-tuned” to the particular stiffness desired.
These above-described isolators provide excellent devices for isolating or reducing the transmission of vibratory motion between an object and the base. However, the components forming the horizontal-motion isolator are often long beam-columns which are loaded (the loading being applied by the supported weight) to approach the beam-column's point of elastic instability. The vertical length of these support beam-columns can be somewhat large resulting in an isolator that may be too tall for particular vibration isolating applications. It would be particularly beneficial, then, if horizontal-motion isolators could be made in a more compact size and shape which may be more suitable for certain vibration isolation applications. However, while a more compact geometry would be beneficial, it is important that the performance of such horizontal-motion isolators not be compromised. Previous solutions have had a practical limit on how short they can be made without negatively affecting the isolator's payload range. The required height to get low frequency isolation with previous solutions can make them not particularly practical for some applications. The geometry of the horizontal-motion vibration isolator/systems of the present invention is such that it reduces the complexity to achieve low resonant frequencies. Accordingly, the present inventions solve the problems of achieving a horizontal-motion isolator having a low vertical height and provides other beneficial features.